Document counters and batchers are well known in the art, being disclosed in such prior patents as Sherman et al U.S. Pat. No. 4,608,704 and DiBlasio U.S. Pat. No. 4,474,365. Electronic control systems for such apparatus are also well known in the art as exemplified by such patents as Jones U.S. Pat. Nos. 4,237,378, 4,015,110 and 3,870,868, as well as the above-identified Sherman et al U.S. Pat. No. 4,608,704.
Various "doubles detection" circuits have been used in such systems to detect the simultaneous multiple feeding of documents. One of the problems that has avoided satisfactory solution in the past is that of distinguishing a single $20 bill that has become dirty through use from a pair of superimposed clean $5 bills. The superimposed $5 bills, if scanned vertically, contain high-density printed matter over a relatively small portion of the scanning line, but are otherwise of low optical density. The single dirty $20 bill, on the other hand, has an optical density whose average approximates that of the superimposed $5 bills, but which is more uniform.
In the systems disclosed in U.S. Pat. Nos. 4,237,378 and 4,608,704, doubles signals are generated if the outputs of path sensors indicate greater than normal optical density for longer than a predetermined period. Such systems are sensitive only to whether the optical density signal is above or below a certain threshold at a given instant, and beyond this are insensitive to the particular signal level. As a result, such systems are susceptible to reading, as doubles, singles whose optical density exceeds the normal level only very slightly for a sufficient period of time.
Other systems, such as shown in Onishi et al U.S. Pat. No. 4,605,926, simply integrate the optical density level and generate an error signal if the integration result lies beyond permissible limits. Although such systems better distinguish between optical density signals having different degrees of peakiness, they cannot differentiate between signals of the same average optical density, such as the signals for the single dirty $20 bill and the superimposed clean $5 bills referred to above.
Furthermore, both types of systems integrate with respect to time, and hence are undesirably sensitive to momentary fluctuations in drive speed.
Other shortcomings of prior-art control systems involve the path sensors themselves. Generally these sensors comprise a light-emitting diode (LED) on one side of the sheet path and a suitable photodetector such as a photodiode or phototransistor receptive to light from the LED. Greater sensitivity in such systems is generally achieved by operating the LEDs at a relatively high output level. Such LED output levels that maximize sensitivity, however, also greatly shorten the useful lifetime of the LEDs. Certain of the systems identified above incorporate the LED and the photodetector into a feedback circuit in which the detector output is maintained at a relatively constant level, thereby reducing the LED output level when no documents are present In certain applications such as measuring optical density, however, it may be desirable for the light incident on the document to remain constant during the scanning period.
Still other problems involve document handlers containing endorser assemblies, which are well known in the art as exemplified by Loftus et al U.S. Pat. No. 4,054,090 and McInerny U.S. Pat. No. 4,004,506. A particular shortcoming of apparatus of this type is that the speed at which the apparatus reliably operates is appreciably lower with the endorser assembly engaged than with the endorser assembly disengaged. Simply limiting the document speed to a speed compatible with endorser operation, however, will unnecessarily limit the efficiency of the apparatus when the endorser assembly is disengaged.